The invention relates to a mirror projection system for use in a step-and-scan lithographic projection apparatus for imaging a mask pattern, present in a mask, on a substrate by means of a beam of EUV radiation, which beam has a circular segment-shaped cross-section.
The invention also relates to a lithographic apparatus for step-and-scan imaging of a mask pattern on a number of areas of a substrate, which apparatus comprises such a mirror projection system.
EP 0 779 528 A2 describes a mirror projection system for use in a step-and-scan lithographic apparatus with which an IC mask pattern is imaged on a number of areas of a semiconductor substrate, using EUV radiation. EUV, extreme ultraviolet, radiation is understood to mean radiation having a wavelength in the range between several nm and several tens of nm. This radiation is also referred to as soft X-ray radiation. The use of EUV radiation provides the great advantage that extremely small details, of the order of 0.1 .mu.m or less, can be imaged satisfactorily. In other words, an imaging system in which EUV radiation is used has a very high resolving power without the NA of the system having to be extremely large, so that also the depth of focus of the system still has a reasonably large value. Since no suitable material of which lenses can be made is available for EUV radiation, a mirror projection system must be used for imaging the mask pattern on the substrate, instead of a hitherto conventional lens projection system.
The lithographic apparatuses currently used in the production of ICs are stepping apparatuses. In these apparatuses, a full field illumination is used, i.e. all areas of the mask pattern are illuminated simultaneously and these areas are simultaneously imaged on an IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area will be positioned under the mask pattern, whereafter this area is illuminated, and so forth until all IC areas of the substrate of the mask pattern are illuminated. As is known, it remains desirable to have ICs with an increasing number of components.
It is attempted to meet this desire not only by reducing the dimensions of these components but also by enlarging the surface areas of the ICs. This means that the, already relatively high, NA of the projection lens system must be further increased and, for a stepping apparatus, the image field of this system must also be further increased. This is practically impossible.
It has therefore been proposed to change from a stepping apparatus to a step-and-scan apparatus. In such an apparatus, a circular segment-shaped sub-area of the mask pattern and hence also such a sub-area of an IC area of the substrate is illuminated, and the mask pattern and the substrate are moved synchronously through the illumination beam, taking the magnification of the projection system into account. A subsequent circular segment-shaped sub-area of the mask pattern is then imaged each time on a corresponding sub-area of the relevant IC area on the substrate. After the entire mask pattern has been imaged on an IC area in this way, the substrate holder performs a stepping movement, i.e. the beginning of a subsequent IC area is introduced into the projection beam and the mask is set to its initial position, whereafter said subsequent IC area is scan-illuminated via the mask pattern. This scan-imaging method may be used to great advantage in a lithographic apparatus in which EUV radiation is used as projection radiation.
The embodiment of the projection system described in EP 0 779 528, intended for use with EUV radiation having a wavelength of 13 nm has an NA of 0.25 at the image side. The annular image field has an inner radius of 29 mm and an outer radius of 31 mm and a length of 30 mm. The resolution of the system is 30 nm and the aberrations and distortions are sufficiently small to form a good image of a transmission mask pattern on an IC area of a substrate by way of a scanning process. This projection system comprises six imaging mirrors, and an intermediate image is formed between the third and the fourth mirror. The mask is situated at one geometric end of the projection system, and the substrate is situated at the other geometric end of this system.
When such a mirror system is used in a lithographic projection apparatus, the small quantity of radiation incident on the substrate presents a problem. This problem is caused by the fact that the mirrors are considerably less than 100% reflective. Each of these mirrors comprises a multilayer structure whose composition is optimized for the relevant wavelength. Examples of such multilayer structures are described in U.S. Pat. No. 5,153,898. With such multilayer structures, maximum reflections can be achieved which are theoretically of the order of 75% but, in practice are currently not larger than 65%. When six mirrors are used, each with a reflection of 68% in a projection system, only 9.9% of the radiation originating from the mask pattern and entering the system reaches the substrate. For a lithographic apparatus, this means in practice that the illumination time should be relatively long so as to obtain the desired quantity of radiation energy on an IC area of the substrate, and the scanning rate should be relatively small for a scanning apparatus. However, it is essential for these apparatuses that the scanning rate is as high as possible and the illumination time is as low as possible, so that the throughput, i.e. the number of substrates which can be illuminated per unit of time, is as high as possible.
It is an object of the present invention to provide a projection system of the type described in the opening paragraph with which this requirement can be met. To this end, the projection system is characterized in that it comprises five imaging mirrors.
An imaging mirror is understood to be a concave (positive) mirror or a convex (negative) mirror which has a converging or a diverging effect and consequently contributes to the imaging by the mirror projection system.
Since only five imaging mirrors are used, the projection system can be manufactured more easily and at lower cost. Indeed, very stringent requirements of accuracy are imposed on an imaging mirror, so that this mirror itself is difficult to manufacture and is expensive. If such a mirror can be omitted from an optical system, this will involve a considerable reduction of cost, also because it will then be easier to assemble this system. A very important advantage of a projection system with five, instead of six, mirrors is that, in principle, the quantity of radiation on the substrate can be raised by approximately 50%. In the given example of 68% reflecting multilayer structures, the quantity of radiation incident on the substrate in a five-mirror system is 14.5% of the radiation entering the projection system. The invention is based on the recognition that, also with five imaging mirrors, a projection system can be realized having a numerical aperture which is by all means acceptable, and has sufficiently small aberrations. With the novel projection system, a circular segment-shaped scanning image spot is formed which is comparable with that formed by the known six-mirror system.
It is to be noted that U.S. Pat. No. 5,153,898 claims a mirror projection system comprising at least three and at most five mirrors. However, it appears from the description of this patent that only three mirrors are used for the actual imaging. In the embodiment using a fourth mirror, this mirror is a flat mirror having the function of giving the imaging beam a different direction so that room is created for the substrate to perform the desired movements. An embodiment with five imaging mirrors is not described. The projection systems described in U.S. Pat. No. 5,153,898 have a small numerical aperture, namely 0.05 or smaller, and all of them are intended for full-field illumination, hence not for use in a scanning apparatus. It is true that U.S. Pat. No. 5,153,898 gives the general remark that the mirror system can be made suitable for a scanning apparatus, but no design is given for a mirror system suitable for this application.
Within the above-mentioned design concept of the novel projection system, there is still some freedom of choice of the parameters NA, magnification and image spot. A preferred embodiment of the system is, however, characterized in that it has a numerical aperture NA of 0.20 and a magnification M of 0.25.
The projection system is preferably further characterized in that it consecutively comprises, from the object plane to the image plane, a concave first mirror, a convex second mirror, a concave third mirror, a convex fourth mirror and a concave fifth mirror, and the object plane is situated between the second and the fifth mirror.
The projection system is preferably telecentric at the image side.
Consequently, no magnification errors can occur upon undesired displacements of the substrate along the optical axis.
The projection system may be further characterized in that the pupil is situated between the first and the second imaging mirror.
The system is then designed in such a way that there is sufficient space to accommodate a diaphragm at that position between the beams extending in opposite directions.
The projection system is preferably further characterized in that all imaging mirrors have aspherical surfaces.
The system is then satisfactorily corrected and has a good imaging quality.
A mirror projection system according to the invention, which is optimal as regards radiation efficiency and build-in space, is characterized in that the object plane is situated between the second and the fifth imaging mirror and in that space for a reflective mask and its holder is reserved between these mirrors.
When this projection system is used in a lithographic apparatus, the dimension of the apparatus in the direction of the optical axis may remain limited because the mask holder is arranged in the projection system in which it can perform the necessary movements via the mask table accommodating the mask holder. The mirror projection system is now divided into a collimator section, consisting of the first three mirrors which jointly a substantially parallel imaging beam, and an objective section receiving this beam. The latter section, consisting of the fourth and the fifth mirror, constitutes the desired telecentric imaging beam. An intermediate image is formed proximate to the second, convex mirror, which intermediate image is very effective for correcting curvatures of the image field. Space for accommodating a diaphragm is available between the first and the second imaging mirror.
Under circumstances, for example, when more space for the mask table is needed, it may be desirable that the mask be situated outside the projection system and at the side of this system other than where the substrate is situated, i.e. the object plane and the image plane are situated at different sides of the projection system. The concept of a projection system with five imaging mirrors can also be used in that case. The adapted embodiment of the projection system is characterized in that a plane mirror is arranged between the first and the second imaging mirror, and in that the object plane is situated outside the projection system.
The plane mirror does not contribute to the actual formation of the image, but only reverses the direction of the radiation from the first imaging mirror. Since the plane mirror can be more easily manufactured and positioned than an imaging mirror, the advantage of a projection system with five imaging mirrors, as compared with a projection system having six imaging mirrors, is maintained.
To achieve the desired optical quality, the circular segment-shaped image field of the projection system with a plane mirror has a slightly smaller width than that in a projection system with five mirrors. An embodiment of the projection system with a plane mirror having an enlarged image field is characterized in that the plane mirror has an aspherical correction surface.
Due to this aspherical surface, the projection system as a whole can be better corrected and, in principle, the circular segment-shaped image field may have an equally large width as the width in the projection system having five imaging mirrors.
The invention also relates to a lithographic apparatus for step-and-scan imaging of a mask pattern, present in a mask, on a number of areas of a substrate, which apparatus comprises an illumination unit with a source for EUV radiation, a mask holder for accommodating a mask, a substrate holder for accommodating a substrate, and a projection system. This apparatus is characterized in that the projection system is a mirror projection system as described above.